HID: wiimote: Register input devices for extensions

Motion+ and regular extensions are physical adapters for the wiimote so create
one input device for each of them. This also allows to enable only opened
extensions and turn unused extenions off to save battery power.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>